Classical computers operate using classical physics principles and include transistors, semiconductors, and integrated circuit technology. In order to achieve greater speed and capability, classical computers increasingly use smaller and smaller wires and logic gates on the order of microns wide. As classical computer chips reach the nanometer scale and logic gates consist of a few atoms, classical limits are approached, and quantum mechanical principles and phenomena begin to dominate. This physical limit presents a barrier to the speed with which computations may be carried out by a classical computer.
Quantum computing utilizes the principles of quantum physics, rather than classical physics, to store and manipulate data, and operates on two principles having no corollary in classical physics: superposition and entanglement. Just as a binary digit, or “bit,” is the basic unit of information in a classical computer, a quantum bit, or “qubit,” is the basic unit of information in a quantum computer. A qubit generally is a system that has two degenerate quantum states. Unlike a classical bit, which exists in one of two states (0 or 1), the qubit can exist in a superposition of both of its degenerate states. As a result, a quantum computer comprised of N qubits can undertake 2N computations in a single step. Thus, as more qubits are added to a quantum computer, the computing power increases exponentially.
The superposition or “coherence” state of a qubit is difficult to maintain because interactions with the surrounding environment cause the qubit to rapidly decay into a classical or “decoherent” state, which destroys the qubit's ability to perform computations. Therefore, a primary obstacle to building a viable quantum computer is maintaining the qubit in its coherent state long enough to do useful work.
Entanglement refers to pairs of particles that have interacted at some point in the past. Entangled particles that are spatially isolated remain related. More particularly, the state of both particles of an entangled pair is always simultaneously determined. For example, measurement of a first particle of an entangled pair collapses the first particle's wave function into a single observable quantity and simultaneously determines the observable state of the second particle of the entangled pair. Pauli's exclusion principle prevents both particles of the entangled pair from occupying the same state. Thus, if one particle of the pair is determined to have a logic 1 state, the other must have a logic 0 state.
Several quantum information processing (QIP) systems for use in quantum computers are known. Each of these systems, however, has distinct disadvantages. One system uses well-established nuclear magnetic resonance (NMR) techniques to store and read information from the degenerate nuclear spin states of molecules in solution. Such a system has been used to complete basic mathematical functions, such as factoring the number 15. However, a NMR-based quantum computer requires a large number of molecules in solution to complete even relatively simple functions, and the system suffers from an attenuated signal-to-noise ratio as the number of molecules increases. Thus, the complexity of calculations that a NMR-based quantum computer is capable of carrying out may be limited.
Another QIP system uses a C60 Fullerene molecule in which an atom or molecule having an unpaired electron is encased, creating an endohedral Fullerene, and encodes data in the spin states of the unpaired electrons using electron spin resonance (ESR) techniques. However, charge transfer from the enclosed atom or molecule to the Fullerene cage often rapidly occurs, which leads to quantum decoherence and loss of the information encoded in the unpaired electron. Charge transfer to the Fullerene cage also limits the make up of atoms and molecules that may be enclosed. In addition, the relatively small size of the Fullerene cavity limits the types of atoms and molecules that may be enclosed. Furthermore, inserting an atom or molecule inside the cavity of a Fullerene molecule is difficult, and the success rate for the uptake of these cargo elements is poor. These factors, coupled with the high cost of the materials needed to fabricate doped Fullerene molecules, limit the potential size and computing power of a Fullerene-based quantum computer.
An alternative QIP system utilizes an electromagnetic ion trap to store and manipulate ions. Information is encoded by manipulating the electronic state of the trapped ion's valence electrons. However, ion trap systems must operate at extremely low temperatures to maintain quantum coherence long enough to be useful, thus requiring an elaborate cooling system.
Other QIP systems make use of “quantum dots” which include small amounts of a semiconducting material enclosed within another semiconducting material. Information is encoded in the quantum dot by manipulating the energy state of particles within the enclosed semiconducting material. Existing QIP systems involve embedding several quantum dots in a solid-state microdisk. However, the excess microdisk material that surrounds the quantum dot contributes to contaminating background radiation and shortened coherence times, which degrades the performance of the system and limits the scale of a quantum dot-based quantum computer.
Semiconductor-based QIP systems typically involve a “top down” assembly approach, and employ some form of lithography and replication. Top down approaches can be time consuming, expensive and wasteful of materials.
Thus, there exists a need for an improved QIP element that avoids the shortcomings of conventional designs.